1. Field of the Invention
The present invention relates to a microelectronic logic gate structure which is convertible in a substantially undectable manner to perform a selected one of two or more logic functions.
2. Description of the Related Art
Intense industrial competition and security in the production of microelectronic components has created a demand for "secure" logic circuits which are difficult to "reverse engineer". Reverse engineering refers to the analysis of a microelectronic circuit integrated device to determine the detailed structure thereof, such so that the device can be copied or otherwise compromised.
A method for protecting hardware integrated circuits or microcircuits against unauthorized copying and/or use is described in U.S. Pat. No. 4,766,516, entitled "Method and Apparatus for Securing Integrated Circuits from Unauthorized Copying and Use", issued Aug. 22, 1988, to F. Ozdemir et al, and assigned to Hughes Aircraft Company, the assignee of the present invention. According to the disclosed technique, at least one additional circuit element that does not contribute toward the function of an integrated circuit (IC), but rather inhibits the proper functioning of the IC in case of an attempted copying, analysis, or other unauthorized use, is fabricated along with the overall IC. The identities of the additional circuit elements are disguised by forming them with the visible appearance of apparent elements, but with physical modifications that are not readily visible to a copyist but cause them to function in a different manner. Such modifications include very narrow open circuit cuts in metallized connection lines, preferably made with a focused ion beam (FIB) or laser beam; disordering the lattice structure or changing the doping level of a semiconductor region, preferably with a FIB; and injecting electrical charge into a semiconductor region, preferably with an electron beam.
The FIB and other types of modifications are relatively undetectable, which is desirable for the intended purpose of deterring unauthorized copying and/or use. However, the additional circuit elements take up valuable space on the microcircuit chips, and do not serve any purpose other than rendering the functional elements operative or inoperative. In addition, processing the additional circuit elements to render the IC operative involves expensive and relatively slow FIB or laser cutting operations, making the method feasible only in applications where large quantity production and/or low fabrication cost are not required.